The picture frames on cinematographic film are conventionally registered with the sprocket holes by register pins. A steady picture display is ensured in such systems as film projection equipment can display each frame using a similar registration mechanism. However, in telecine and film writers for example, the film conventionally moves through the film gate at a steady speed and thus register pins cannot be inserted.
In conventional telecine equipment, the cinematographic film is positioned in the film gate by pressing one edge against the guide skid as illustrated schematically in FIGS. 1 and 2. As the picture frames are located relative to the sprocket holes and not the film edge, a problem arises when the edge of the film is uneven and not accurately positioned relative to the sprocket holes. In such an instance, the frame with the picture image will wander according to the irregularities in the film edge.
The CRT beam scans the same area of the film gate for each frame. Thus, if the frame is slightly misplaced on the film, its actual position will not coincide exactly with the CRT scan. This will have the result that the displayed image will weave slightly from side-to-side. Because film manufacturers register picture frames with respect to sprocket holes, little care is used in maintaining the taughtness of the film edge, thereby increasing film weave.
It is desirable to minimize this film instability effect, and various methods for so doing have been proposed. For example, U.S. Pat. No. 4,104,680, teaches moving the CRT scan to compensate for the film motion and the error is measured by optical detection of the sprocket hole.
A disadvantage of such a system is that further errors may be caused by the presence of dirt or dust in the sprocket hole.